1. Field of the Invention
The present invention generally relates to a circuit for detecting remaining battery capacity and particularly to a circuit for detecting remaining battery capacity in which the remaining battery capacity is detected based on a charge and discharge current of the battery.
2. Description of the Related Art
In recent years, lithium ion batteries have been installed on mobile devices such as digital cameras. The lithium ion batteries are generally considered to be difficult to detect remaining battery capacity based on a battery voltage thereof. Accordingly, a method for measuring remaining battery capacity is employed in which the capacity is measured by integrating a charge and discharge current of the battery (refer to Patent Document 1).
For example, it is very important for digital cameras to reduce waiting time as much as possible before the cameras are ready for shooting from power-on. It is known that the time required to be able to take a picture upon power-on is reduced in proportion as consumption current upon power-on is increased.
When a circuit for detecting remaining battery capacity is used in which a charge and discharge current is integrated, if a large consumption current upon power-on is not detected, a margin of error of remaining battery capacity is increased. This is due to the fact that the circuit for detecting remaining battery capacity consumes current per se and the circuit for detecting remaining battery capacity intermittently measures the current so as to control the current consumption.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-174534
FIG. 8 is an illustration of operations of an example of a conventional circuit for detecting remaining battery capacity. In FIG. 8, tm 11 and tm 12 indicate measurement time, Δt 11 and Δt 12 (Δt 11>Δt 12) indicate measurement intervals.
For example, while a load is powered off, measurement is conducted at relatively long measurement intervals Δt 11 (several minutes, for example) so as to reduce consumption current in the circuit for detecting remaining battery capacity because a change of the consumption current and the consumption current per se are small. By contrast, upon powering on the load, measurement is conducted at relatively short measurement intervals Δt 12 (several seconds, for example) because the consumption current is greatly changed in accordance with a status of the load.
When the load is powered on at time t 10 as shown in FIG. 8, remaining capacity is detected at measurement intervals Δt 12 from time t 11 after the measurement interval Δt 11 has elapsed from the last measurement time tm 11. In accordance with this, it is impossible to detect relatively large current consumption upon powering on the load shown in slashes in FIG. 8 depending on measurement time tm 11. Thus, when powering on and off is repeated, it is impossible to accurately detect remaining battery capacity.
In order to solve this, measurement intervals may be reduced as shown in FIG. 9. FIG. 9 is an illustration of operations of another example of a conventional circuit for detecting remaining battery capacity. In FIG. 9, tm 21 and tm 22 indicate measurement time, Δt 21 and Δt 22 (Δt 21>Δt 22) indicate measurement intervals.
In FIG. 9, measurement intervals Δt 21 while the load is powered off (several tens of seconds, for example) are set to be shorter than the measurement intervals Δt 11 while the load is powered off shown in FIG. 8. In accordance with this, upon powering on the load at time t 20 as shown in FIG. 9, remaining capacity is detected at measurement intervals Δt 22 from time t 21 after the measurement interval Δt 21 has elapsed from the last measurement time tm 21. Accordingly, although current consumption upon powering on the load shown in slashes in FIG. 9 is not detected, a margin of error of remaining battery capacity is reduced in comparison with FIG. 8.
However, the method shown in FIG. 9 is problematic in that current consumption is increased in the circuit for detecting remaining battery capacity because the measurement intervals Δt 21 while the load is powered off are short.
Thus, conventional circuits for detecting remaining battery capacity has a problem in that it is impossible to measure large current consumption upon powering-on, for example, depending on a process time for measuring remaining capacity and it is impossible to conduct accurate detection of remaining capacity.